This report is based upon results obtained with 127 developing White Leghorn embryos subjected to varying times of a gaseous mixture of 5 percent oxygen and 2 percent carbon dioxide (hypoxia/hypercapnea). Of the exposed embryos 67 survived to 14 days or more (roughly equivalent to 7 month fetuses). Control survival rate is 74 percent (27/37 embryos). Still in the incubator at this writing are 113 embryos, including 53 experimentals and 60 controls. Several initial runs are not included in this report because of a low control survival rate, as initial problems of securing viable embryos and workable incubation methods had to be overcome. The experiments described in this report are being continued with special emphasis on the development of a dosage (or the use of earlier embryonic stages) to produce a higher survival rate. The author is also concentrating upon the development of a dissecting technique which will make a more rapid analysis of congenital cardiac defects possible, the time saved to be devoted to histochemical and cytological studies of the developing heart. The more quantitative studies of hemodynamic aspects of the effects of hypoxia and hypercapnea upon cardiac development are scheduled for next year. These studies will include electrocardiographic analyses of early and post-natal stages of heart development. A study of the development of the ECG in the turtle embryo, in addition to the experiences here at the Institute will aid in this project.